The present invention relates to a digital circuit. U.S. Pat. No. 5,144,168 discloses a digital circuit with an input for supplying an input signal through a first switching element. The digital circuit further has an activation input via which the circuit can be set to an activated state and a deactivated state, a first output, a second output and a logic unit. The logic unit is connected on the input side to the two outputs of the circuit and is connected on the output side to a control connection of the first switching element. The logic unit switches the first switching element to a conductive state when there is a first logic level at the two outputs of the circuit, and the logic unit blocks the first switching element when there is a second logic level at one of the two outputs.
It is accordingly an object of the invention to provide a digital circuit that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which, after being set to an activated state, outputs an input signal in a non-inverted state and in an inverted state to corresponding outputs and which, after its activation, permits a favorably timed decoupling from the input signal as soon as the level of the latter has been sensed.
With the foregoing and other objects in view there is provided, in accordance with the invention, a digital circuit, containing a switching element having a control terminal and an input for receiving an input signal and connected to the switching element, the input supplying the input signal to the switching element. An activation input receives an activation signal for setting an activated state and a deactivated state. Two output points including a first output point and a second output point are provided. A logic unit has inputs connected to the two output points and an output connected to the control terminal of the switching element. The logic unit switches the switching element to a conducting state if there is a first logic level at the two output points. The logic unit switches the switching element to a non-conducting state if there is a second logic level at one of the two output points. A circuit unit is connected to the activation input, the switching element, and the inputs of the logic unit. In the activated state, the first output point outputs in a non-inverted form a level of the input signal which the input signal has directly before the setting of the switching element to the non-conducting state. In the activated state, the second output point outputs in an inverted form the level of the input signal which the input signal has directly before the setting of the switching element to the non-conducting state. And in the deactivated state, the first logic level is output at both of the two output points.
The digital circuit according to the invention has the input for supplying the input signal via the first switching element and also the activation input, by which it can be set to an activated state and a deactivated state. Furthermore, it has a first output, at which, in the activated state, it outputs in a non-inverted form that level of the input signal which the latter has directly before a blocking of the first switching element, and a second output, at which, in the activated state, it outputs in an inverted form that level of the input signal which the latter has directly before the blocking of the first switching element. In the deactivated state, it outputs a first logic level at both outputs. In addition, the digital circuit has the logic unit, which is connected on the input side to the two outputs of the circuit and which is connected on the output side to a control connection of the first switching element. The logic unit switches the first switching element to a conductive state when the first logic level is present at both outputs of the circuit. The logic unit sets the first switching element to a non-conducting state (blocking) when a second logic level is present at one of the two outputs.
Consequently, as long as the digital circuit is deactivated, the circuit outputs the same logic level at both of its outputs, whereby the first switching element is switched to the conductive state via the logic unit. Therefore, during this time period, the input signal at the input of the digital circuit is connected through the first switching element to further components of the digital circuit. The level of the input signal in this case does not have any effect on the output signals of the digital circuit. As soon as the digital circuit is set to the active state via its activation input, the signals at both the outputs of the digital circuit are complementary to each other. Which level the two output signals have is dependent on the level of the input signal at the change to the activated state. By monitoring the output signals, the logic unit detects from their different levels that the digital circuit is in the activated state and that the currently applicable level of the input signal was output to the first output in a non-inverted form and to the second output in an inverted form. At this point in time, the logic unit puts the first switching element in a non-conducting state, so that subsequently further changes in the level of the input signal do not have any effect on the output signals. The latter maintain their level until the digital circuit is set to the deactivated state again, whereby a resetting of the output signals to the first logic level takes place.
The invention has the advantage that the setting of the first switching element to the non-conducting state takes place at the optimum time, since it is carried out on the basis of the change in level of the output signals. Consequently, it is immaterial how much time passes between the activation of the digital circuit and the output of the level of the input signal in a non-inverted or inverted form. There is consequently no longer any necessity to measure this time period and reproduce it by a transport delay unit that could alternatively be used for the blocking of the first switching element. Since the digital circuit carries out the blocking of its first switching element in an automatic and time-optimized manner, there is no longer any necessity for such a transport delay unit and the associated development effort with respect to its dimensioning.
According to a development of the invention, the digital circuit has a holding unit with a feedback branch, which prevents any change in the levels at the outputs of the circuit after the blocking of the first switching element for as long as the circuit is activated. In addition, it has a second switching element, which is disposed in the feedback branch and has a control input, which is connected to the output of the logic unit. The logic unit blocks the second switching element when the first logic level is present at both outputs of the circuit, while it switches the second switching element to the conductive state when the second logic level is present at one of the two outputs.
The first switching element and the second switching element are consequently respectively set to opposite switching states. It is ensured by the second switching element that the feedback branch, and consequently the holding unit, is only active when the two outputs of the digital circuit already have different levels. Only then is the first switching element blocked, so that the input signal no longer has any influence on the levels of the output signals. By closing the second switching element, the state of the output signals is held. However, as long as the first switching element is switched to be conducting , the second switching element is blocked, so that no leakage currents flow via the then interrupted feedback branch. Consequently, the power consumption of the digital circuit is reduced.
In accordance with an added feature of the invention, the circuit unit has a holding unit with a feedback branch which prevents any change in the levels at the two output points after switching the switching element to the non-conducting state for as long as the activated state is set. The holding unit has a further switching element with a control input disposed in the feedback branch and the control input is connected to the output of the logic unit. The logic unit switches the further switching element to the non-conducting state if the first logic level is present at both of the output points. The logic unit switches the further switching element to the conducting state if the second logic level is present at one of the two output points.
In accordance with another feature of the invention, the circuit unit, includes an inverter having an input connected to the switching element; a first NAND gate having a first input connected to the inverter, a second input connected to the activation input, and an output being the first output point; and a second NAND gate having a first input connected directly to the switching element, a second input connected to the activation input, and an output being the second output point.
In accordance with a further feature of the invention, the logic unit has a NOR gate.
In accordance with a concomitant feature of the invention, the feedback branch is disposed between the first output point and the input of the inverter.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a digital circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.